Thanks for the additional input.
This looks like a bug that shows in situations where no sender name is present in the email header of the original email.
The expected behaviour would be that WP Mailster pulls in the name from the user database as a backup information, however it does not do it right now.
I will see that we can get this resolved soon and yes, you can use your workaround for now.
About the “References” header:
Email clients like (e.g. GMail or Apple Mail, …) will include the “References” header of the original email in any answer (when you hit reply).
Of course this does NOT mean that any of the email address in “References” will get a copy, they don’t.
Why are there emails ending with @brandt-solutions.de?
Well, this helps WP Mailster to recognize the email thread that replies to conversations/threads are written to.
Part of the email address is the thread ID and part of it is a message specific ID.
We could also write it like <[email protected]>
but we preferred to use an existing domain.